This invention relates to a sintered alloy body subjected to thermal refining of the surface which is effective as a substrate of a coated sintered alloy part such as a cutting insert of cutting tools or a wear resistant part of wear resistant tools and to a method for making the same.
The so-called coated sintered alloy such as cemented carbides coated with thin layers of highly wear resistant materials such as TiC, TiCN, TiN, Al.sub.2 O.sub.3, etc., is endowed with both toughness from the cemented carbide substrate and excellent wear resistance from the coated film, and has been provided widely for practical uses.
The above coated layer, while being excellent in wear resistance, is on the other hand extremely brittle, and therefore cracks are liable to be formed in said coated layer during service, and there was a problem that the cracks were expanded even to the substrate to develop a breakage in the cutting edge. As an excellent prior art proposed for solving this problem, there is Japanese Provisional Patent Publication No. 87719/1979 (which corresponds to U.S. Pat. No. 4,277,283), and this has been already practically utilized.
This prior art discloses a cemented carbide comprising hard phase having B-1 type crystal structure of carbonitride (hereinafter called .beta. phase), another hard phase of WC, and a binder phase of an iron group metal, in which the .beta. phase so migrates from the surface layer of from 5 to 200 microns of the cemented carbide body that the amount of the .beta. phase in the surface layer is less than in the inside, or the surface layer is free of the .beta. phase. And it is stated that the migration of the .beta. phase occurs to the cemented carbide when a green compact comprising the B-1 type carbonitride, WC and an iron group metal is partially denitrified at the surface of the green compact during vacuum sintering. Therefore, the green compact in this prior art indispensably has to contain some nitrogen.
The phenomenon of the migration of .beta. phase from the surface layer of the cemented carbide containing nitrogen was studied in detail by Dr. Hisashi Suzuki, professor of University of Tokyo at that time ("Journal of The Japan Society of Powder and Powder Metallurgy", vol. 29, No. 2, pp. 20-23) and it is shown that the migration of .beta. phase from the surface of the cemented carbide occurs along with denitrification during vacuum sintering.
As mentioned above, the .beta.-migrated cemented carbide has been utilized as a substrate of the coated hard alloy part. However, when the .beta.-migrated cemented carbide according to this prior art was used as a substrate of the coated hard alloy part, it was still found to be insufficient in tool failures such as breakage and wear, as shown below.
FIG. 1 is cited from the drawing described on p. 302 in "Sintered Cemented Carbide and Sintered Hard Material" edited by Dr. Suzuki (Maruzen). As can be seen from the graph in FIG. 1, the migration of .beta. phase is surely realized by the prior art. However, to observe the distribution of the binder metal Co, it is known that the relative concentration of the binder phase at the outermost surface is rather the same level as, or even lower than, the average concentration in the inside. Accordingly, as a matter of course, when such .beta.-migrated cemented carbide with binder-metal-poor outermost surface is used as a substrate for the coated hard alloy part, the effect of inhibiting development of cracks generated in the brittle film to the substrate will be cancelled.
Further, such a coated hard alloy part in which the substrate comprises the .beta.-migrated cemented carbide is significantly disadvantageous when the coated film was peeled off or the coated film was worn away, namely, when the surface of the substrate had been exposed, because severe cratering occurs on the rake face of the cutting tool for lack of .beta. phase in the surface layer of the substrate. It has been well known that the .beta. phase is a strong cratering-resistant ingredient in cemented carbide.
Another prior art pertinent to the present invention has been disclosed in U.S. Pat. No. 4,610,931. This prior art presents a cemented carbide with a binder-enriched surface.
According to the specification of the above prior art, the cemented carbide with binder-enriched surface can be formed, preferably, for example, through the following process: milling and blending WC powder, Co powder and TiN powder; then compacting the blended powder into a desired shape; finally sintering in vacuum furnace the compact so as to transform the TiN to its carbide. According to FIGS. 2 and 3 of the patent, the cemented carbide made by this patent has a characteristic in the relative concentrations of binder phase and .beta. phase the same in the .beta.-migrated cemented carbide mentioned above. Therefore, the cemented carbide with binder-enriched surface according to the patent has the same disadvantages described in the case of .beta.-migrated cemented carbide above.